Hi,
Unless otherwise stated, when we talk about frequency limits we talk about sine waveform. But if you define "square wave" that´s O.K., too.
Now square wave consists of the fundamental frequency plus it´s overtones. So in a (upper) frequency limited system, the overtones become more attenuated than the fundamental. Thus the square wave becomes distorted. The output shape will differ from a clean square wave.
Now you "defined" the waveform. But still you did not define the "maximum". How do you define this limit?
For a sine it´s more simple: You usually do this by the attenuation.
Example:
For a 1st order low pass filter fc is defined when the amplitude drops to 71% (-3dB) of of input amplitude. So a 1kHz LPF still will have 2kHZ, 10kHz, 100kHz at it´s output .... but attenuated (more and more with rising frequency).
There is no (meaningful) frequency with zero output.
With square waves this is more difficult, since - due to the overtones the - shape depemds on the attenuation of each overtone (individually) and additionally on the phase shift.
I´m quite experienced with signal processing, measurements and so on, but I don´t know a general definition when distorted a square wave is considered to be "no square wave" anymore.
*****
Regarding power lines:
The limit does not depend on applied voltage. Disspation is defined by I x I x R_trace.
Klaus